1. Field of the Invention
The invention relates to a method of navigating a magnetic object within an object to be examined which is exposed to a magnetic field. The invention also relates to a magnetic resonance (MR) device which includes a main field magnet system for generating a stationary, static magnetic field and a medical instrument, notably a catheter or a flexible endoscope, which is to be introduced into an object to be examined, a magnetic object being provided in or on the instrument.
2. Description of Related Art
A method of this kind is known from WO 9603795 A1. Therein, a passive magnetic object (a permanent magnet) is moved to given positions within the body of a patient by means of temporally and spatially varying magnetic fields which are generated by a plurality of notably superconducting coils. Therein, the motion is observed by means of X-rays.
An MR device of the kind set forth is disclosed in U.S. patent application Ser. No. 08/754,360. Therein, a microcoil attached to a catheter is used to observe the motion of the catheter within an object to be examined, for example by forming MR images from signals received from the microcoil.
It is an object of the invention to provide a method of navigating a magnetic object which is particularly suitable for magnetic resonance tomography. It is also an object of the invention to provide an MR device which is suitable for navigating a magnetic object.
The object in respect of the method is achieved in that the object has a controllable magnetic moment and that the direction of movement of the object is determined by control of the magnetic moment.
According to the invention, an active magnet (an electromagnet) is used generate the controllable magnetic moment which can notably be switched on and off. According to the known method, however, use is made of a passive magnet having an invariable magnetic moment; this is not suitable for MR tomography. The control of the direction of movement of the object utilizes the physical effect that an object having a magnetic moment in a magnetic field is subject to a mechanical moment whose direction and strength are dependent on the direction and the strength of the magnetic moment and of the magnetic field. In order to guide the object in a desired direction, according to the invention, a magnetic moment of suitable magnitude and direction is adjusted so that a mechanical moment occurs in the desired direction and with the required strength in the magnetic field present.
In preferred embodiment, each of three coils in the object can be individually supplied with a respective current, so that a magnetic moment can be adjusted in any arbitrary direction, thus enabling the object to be guided in any direction. The coils can receive a current simultaneously or consecutively.
A further embodiment of the invention is simple and space-saving. Because a magnetic moment cannot be adjusted in any direction when only one or two coils are used, this embodiment is provided with means, notably mechanical means such as joints and Bowden cables, in order to change the position of the individual coils or the coil system relative to the external magnetic field, and hence to change also the direction of the magnetic moment of a coil or the coil system. Each coil can again be supplied with a current individually or all coils can be supplied with a current simultaneously, and for an adjustable period of time, thus enabling adjustment of the desired magnetic moment.
The method according to the invention is preferably used in an MR device, the magnetic field then being the stationary, static magnetic field of the MR device. This magnetic field is notably also uniform and has a high magnetic field strength so that, even when use is made of a small coil, an adequate mechanical moment can be produced so as to guide the object in the magnetic field exclusively by means of the acting mechanical moment.
The method according to the invention is particularly suitable for moving a medical instrument, notably a catheter or a flexible endoscope, in an object to be examined, for example in the vascular system of the head of a patient. The mechanical instrument is then advanced, for example by hand, whereas a change of direction is realized by the method according to the invention.
In a further embodiment of the invention the position of the object can also be determined. This embodiment is used notably in an MR device and when the object is implemented as a coil system.
The object has a controllable magnetic moment and there are provided means for controlling the magnetic moment and for navigating the magnetic object within the object to be examined. These means may include, for example a suitable control unit and suitable current sources for supplying individual coils, preferably constituting the magnetic object, with currents for adjustment of a magnetic moment acting on the object.